The valves of the shell are not symmetrical ; the left oim, by 

 which the oyster is always attached to its support, is deep and 

 spoon shaped, while the right one is nearly flat. 



'llie work of compressing the ligament and closing the shells 

 is done by a large, powerful muscle (Plate 1, Fig. 1, m), which 

 runs across the body between the shells, and is fastened to their 

 inner surface over the dark-colored spots that are always found 

 on the inside of empty oyster shells. This muscle, and some- 

 times the muscle scar on the shell, is called the "eye" by the 

 Louisiana oyster fishermen. 



The shell is lined by a thin membrane, the mantle (Plate 1, 

 Fig. 1, mt), which folds down on each side, and may be com- 

 pared to the leaf next the cover on each side of the book. The 

 next two leaves of each side represent the gills (Plate I, Fig. 1, 

 g), the so-called "beard" of the oyster, which hang down like 

 leaves into the space inside the two lobes of the mantle. The 

 remainder of the book may be compared to the body of visceral 

 mass of the oyster. 



In order to examine the soft parts of an oyster, the right 

 (.flat) valve should be carefully removed by cutting the adductor 

 muscle and breaking the ligament, thus exposing the right side 

 of the body. The surface of the body is covered by a fold of 

 tissue, the mantle (mt), which is attached to the body over the 

 greater part of its surface, but hangs free around nearly the 

 whole circumference. By tearing away the mantle, the gills will 

 be exposed. These are four folds of tissue, which occupy the 

 lower half of the mantle cavity and extend nearly the whole 

 length of the body. The space above the posterior (outer) ends 

 of the gills is occupied by the adductor muscle, or "eye." Be- 

 tween this muscle and the hinge, the space above the gills is 

 occupied by the body mass, made up of licrht-pnlnrod reproduc- 

 tive organs and dark-colored digestive orirans bound totrether 

 in one continuous mass. 



Between the muscle and the visceral mass will be seen a 

 transparent, crescent-shaped space, the pericnrdmm, and, if the 

 delicate membrane which forms its walls be carefully cut away, 

 the heart (Plate I, Pier. i. ve and au). may be found without any 

 •diflficulty lying in this cavity and pulsating slowly. 



In front of the gills (that is between them and the hinged. 



